1. Field of the Invention
The present invention relates to an optical displacement measuring device for detecting the relative movement position of a movable portion such as a tooling machine, a semiconductor manufacturing device, or the like. More specifically, the present invention suppresses influence of displacement of a diffraction grating to other than a direction where a movement position is detected with reflecting optical systems for irradiating two diffracted beams diffracted at the diffraction grating on the diffraction grating again by setting the focal lengths of imaging devices of the respective reflecting optical systems to the same and disposing the diffraction grating and a reflector around the focal positions of the imaging elements.
2. Description of the Related Art
Heretofore, an optical displacement measuring device employing a diffraction grating has been employed as a device for detecting the relative movement position of a movable portion such as a tooling machine, a semiconductor manufacturing device, or the like (e.g., see Japanese Unexamined Patent Application No. 2000-81308 and Japanese Examined Patent Application No. 63-311121).
With an optical displacement measuring device according to the related art, a coherent beam emitted from a coherent light source is divided into two coherent beams at a beam splitter or the like, and these are irradiated on a diffraction grating. The two coherent beams irradiated on the diffraction grating are diffracted at the diffraction grating to generate two one-time diffracted beams. The two one-time diffracted beams are reflected at reflecting optical systems, are returned on the same optical paths, and are irradiated on the diffraction grating again.
The two one-time diffracted beams irradiated on the diffraction grating are diffracted at the diffraction grating to generate two two-time diffracted beams. The two two-time diffracted beams are returned on the same optical paths as the two coherent beams and are input to the beam splitter. The two two-time diffracted beams input to the beam splitter are overlapped at the beam splitter, and an interference beam obtained by causing the two two-time diffracted beams to interfere with each other is imaged on a light receiving element.
With the optical displacement measuring device having such a configuration, the diffraction grating moves to a grating vector direction according to movement of a movable portion, thereby causing a phase difference between the two two-time diffracted beams. The optical displacement measuring device detects an interference signal by causing the two two-time diffracted beams to interfere with each other, and obtaining the phase difference between the two two-time diffracted beams from the interference signal to detect the movement position of the diffraction grating.
With an optical displacement measuring device according to the related art, two coherent beams are imaged on the diffraction grating, and also the one-time diffracted beams diffracted at the diffraction grating are irradiated on reflectors by imaging elements as collimated beams. The one-time diffracted beams diffracted at the reflectors are returned on the same optical paths and are imaged on the diffraction grating by imaging elements.
Thus, with an optical displacement measuring device according to the related art, the one-time diffracted beams diffracted at the diffraction grating are returned on the same optical paths as those at the time of inputting. Also, the imaged positions on the grating plane of the diffraction grating are not changed, and the optical axes of two-time diffracted beams generated by the one-time diffracted beams being diffracted are not shifted. Also, no change in the optical path lengths occurs. Accordingly, even if the diffraction grating is subjected to posture change, the interference signal between the two two-time diffracted beams is not readily disordered, thereby enabling stable detection.